1. Field of the Invention
The present invention broadly relates to LC filters, and more particularly, relates to LC filters included in mobile communication devices, such as cellular telephones, and various electronic devices.
2. Description of the Related Art
Generally, a conventional high-frequency band-pass LC filter includes parallel LC resonant circuits, having the same impedance and coupled by mutual inductance coupling (M-coupling) or capacitive coupling. FIG. 5 is an exploded perspective view of a conventional LC filter, and FIG. 6 is an electrical equivalent circuit of the conventional LC filter.
Referring to FIG. 6, a conventional LC filter 100 includes an input-side parallel LC resonant circuit 1 and an output-side parallel LC resonant circuit 2, which are capacitively-coupled by a coupling capacitor C3. Referring now to FIG. 5, on ceramic sheets 5, the LC filter 100 includes an inductor pattern 7 of an inductor L1 and capacitor patterns 9a and 9b of a capacitor C1, which form the input-side parallel LC resonant circuit 1. The LC filter 100 further includes, on the ceramic sheets 5, an inductor pattern 8 of an inductor L2 and capacitor patterns 10a and 10b of a capacitor C2, which define the output-side parallel LC resonant circuit 2. The LC filter 100 also includes a capacitor pattern 11 of the coupling capacitor C3 on the ceramic sheet 5. These sheets are stacked and monolithically baked to define a surface-mounting electronic device. The inductor pattern 7 of the inductor L1 is connected to an input terminal 101 (see FIG. 6) at a leading portion 7a extending from the center thereof. Similarly, the inductor pattern 8 of the inductor L2 is connected to an output terminal 102 (see FIG. 6) at a leading portion 8a extending from the center thereof.
The inductor pattern 7 of the inductor L1 and the inductor pattern 8 of the inductor L2 have the same pattern configuration and the same size. The capacitor patterns 9a and 9b of the capacitor C1 and the capacitor patterns 10a and 10b of the capacitor C2 have the same pattern configuration and the same size. Therefore, the ratio L/C of the inductance of the inductor L1 to the capacitance of the capacitor C1, which define the input-side parallel LC resonant circuit 1, is equal to the ratio L/C of the inductance of the inductor L2 to the capacitance of the capacitor C2, which define the output-side parallel LC resonant circuit 2. Hence, the input impedance and the output impedance of the LC filter 100 are equal.
Since the input impedance and the output impedance of the conventional LC filter 100 are the same, when respective external circuits connected to the input side and the output side have different impedances, it is necessary to connect impedance matching circuits to the input side and the output side, respectively, in order to match the impedance with the external circuits. It is therefore necessary to provide an area on the LC filter to accommodate the impedance matching circuits. Further, characteristics of the LC filter 100 may vary due to the effect of the connected impedance matching circuits.
The inductor patterns 7 and 8 are located adjacent to each other on the same ceramic sheet 5. Therefore, electromagnetic coupling of the inductor patterns 7 and 8 must also be considered. When the inductances of the inductors L1 and L2 are changed by changing the configurations of the inductor patterns 7 and 8 due to a change in a design specification or other reason, the electromagnetic coupling between the parallel LC resonant circuits 1 and 2 is also changed. This requires a re-examination of design constants.
To overcome the problems described above, preferred embodiments of the present invention provide an LC filter having desired input/output impedances in which design changes can easily be made.
To this end, according to one preferred embodiment of the present invention, an LC filter preferably includes a plurality of parallel LC resonant circuits. The ratio of the inductance of a first inductor relative to the capacitance of a first capacitor, which define an input-side parallel LC resonant circuit, is different from the ratio of the inductance of a second inductor relative to the capacitance of a second capacitor, which define an output-side parallel LC resonant circuit. Preferably, the first and second inductors of the parallel LC resonant circuits each include inductor patterns having a layered structure.
With this arrangement, the input-side parallel LC resonant circuit and the output-side parallel LC resonant circuit each have resonant frequencies approximately equal to the center frequency of the LC filter. Each of the resonant frequencies is determined by the product of the inductance of the inductor and the capacitance of the capacitor, which define the parallel LC resonant circuit. The ratio of the inductance of the first inductor relative to the capacitance of the first capacitor, which define the input-side parallel LC resonant circuit, is different from the ratio of the inductance of the second inductor to the capacitance of the second capacitor, which define the output-side parallel LC resonant circuit. Therefore, the input impedance and the output impedance of the LC filter may be set to the desired impedances.
Preferably, the parallel LC resonant circuits, which are adjacent, are coupled by a coupling capacitor. The inductor patterns constituting each of the first and second inductors in the adjacent parallel LC resonant circuits may be disposed, in a thickness direction of the layered member, with capacitor patterns defining the coupling capacitor and capacitor patterns defining each of the first and second capacitors in the adjacent parallel LC resonant circuits being disposed therebetween. Accordingly, mutual inductance coupling between the inductors in the adjacent parallel LC resonant circuits is minimized. Therefore, even when the configuration of the inductor patterns are changed to change the inductances of the inductors, characteristics of the LC filter are minimally influenced.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiment of the present invention below with reference to the attached drawings.